1. Field of the Invention
The present invention relates to the removal of blocking effects in images previously compressed and decompressed. More particularly, the present invention relates to the removal of blocking effects in images compressed in accordance with MPEG, JPEG and other DCT based image coding formats.
2. Art Background
The need to compress digital image data, whether static or dynamic (i.e., video images) images has dramatically increased. One reason is the dramatic popularity of multimedia computer systems and software. Another reason is switch to digital broadcasts of station programming. One example of the latter is direct satellite broadcasts, such as DSS.TM.. In order to minimize the bandwidth required to transmit broadcasts of programs or to minimize the amount of storage space required to store certain images, compression techniques are utilized. Thus, the image data is transmitted or stored in a compressed format and prior to the display of the image, the image data is decompressed. Examples of widely utilized compression algorithms are those that comply with the MPEG (Moving Pictures Experts Group) and JPEG (Joint Picture Experts Group) standards.
Many compression processes, including those that comply with MPEG or JPEG standards, utilize transform coding. In a transform coding process, an image is divided into small blocks. The transform of each block is taken, the coefficients of which are quantized in accordance with a determined quantization factor q. The most popular transform is the discrete cosine transform (DCT).
However, one negative effect of the process is referred to as the "blocking effect". By dividing the image into blocks prior to encoding, discontinuities (referred to as blocking effects) between adjacent blocks occurs through the encoding and decoding processes. This is represented in a displayed decompressed image by clear jumps between colors or greyscales as opposed to a smooth change.
Pre-processing and post-processing techniques are utilized to minimize blocking effects. Pre-processing techniques dictate that the originator of the image data must perform certain steps to minimize blocking effects. Post-processing techniques, although logistically better as the correction is performed after decompression, has its problems. For example, one of the simplest techniques is to process the decompressed image data through a low pass filter. Although the blocking effects are decreased, the sharpness of the displayed image is negatively affected.